??AOL??
Just one other thing: if either of you uses AOL as your isp, this may be summat to do with it.
I'm too indolent to move from AOL so had exactly the same problem as you. In the end I discovered that one of the many issues with AOL is that it sometimes won't happily deal with attachments to an HTML file, but with text files it's a pussycat. Could this explain the inconsistency you experience?
A couple of test files could give an answer? Wotcherfink?
Jim